In recent years, wireless power supply for wirelessly supplying power has been actively studied. As methods for wireless power supply, methods using a magnetic field are available. The methods using a magnetic field are roughly classified into electromagnetic induction methods and magnetic-field resonance methods.
Electromagnetic induction methods are already being widely used. In the methods, the degree of coupling between a power transmission apparatus which transmits power and a power reception apparatus which receives power is very high, and power can be supplied highly efficiently.
Magnetic-field resonance methods are methods in which a resonance phenomenon is actively used, and are characterized in that the number of lines of magnetic induction shared by a power transmission apparatus and a power reception apparatus may be small.
Both of electromagnetic induction and magnetic-field resonance are methods for supplying power by using a magnetic field. Thus, the power transmission apparatus includes a power transmission coil, which is a coil for transmitting power by using a magnetic field, and the power reception apparatus includes a power reception coil, which is a coil for receiving power by using a magnetic field.
With the magnetic coupling between the power transmission coil and the power reception coil, power is supplied from the power transmission apparatus to the power reception apparatus.
If foreign matter capable of receiving energy from a magnetic field, such as metal, is between the power transmission coil and the power reception coil which are magnetically coupled to each other, an eddy current flows through the foreign matter and thereby heat is generated. Accordingly, energy is consumed and power supply efficiency decreases.
Under such circumstances, various methods for detecting foreign matter existing between a power transmission apparatus and a power reception apparatus have been suggested.
For example, PTL 1 describes the following technology. The current on a power transmission apparatus side is monitored, and, if the current is an eddy current, it is determined that foreign matter has been detected, and transmission of power from the power transmission apparatus is stopped.